1. Technical Field
The present invention relates to a reference sensor; and more particularly to a reference sensor having a fiber Bragg grating therein.
2. Description of Related Art
Fiber Bragg gratings have found many uses, one of which is the use thereof as wavelength reference elements. They possess the capability to provide an extremely accurate and stable optical signal centered about a well defined wavelength region. This property permits them to act as dependable references for use in such applications as instrumentation designed to accurately read optical signals.
The present invention provides a fiber Bragg grating reference sensor that will permit the use of an optical fiber having a fiber Bragg grating therein as a precise reference sensor.
A fiber Bragg grating is inherently sensitive to parameters such as temperature and strain, both of which will shift the resonance condition within the device and, therefore, affect the reflected signal from the grating. The temperature sensitivity can be on the order of 10 picometers per degree Celsius. To utilize the fiber Bragg grating as a reference it must be isolated from any changes in these parameters, or they must be controlled and measured. In the present invention, the fiber Bragg grating is completely isolated from strain but allowed to drift freely with temperature. The temperature can then be measured and a knowledge of the fiber Bragg grating response to temperature can be utilized to determine the exact wavelength thereof. With this approach, the temperature of the fiber Bragg grating is measured accurately and precisely, but without affecting the fiber Bragg grating itself, or the thermal expansion characteristics of the fiber Bragg grating. If the fiber Bragg grating is potentially restricted in movement as the temperature thereof changes, this may induce a strain over the fiber Bragg grating and cause a change in the wavelength reading.
The fiber Bragg grating itself must be strain relieved to prevent any strain effects, and this can be achieved by anchoring both outer ends of the grating to a glass element which has the same coefficient of thermal expansion (CTE) as the fiber Bragg grating itself. The anchoring will ensure that the glass strain relief will not induce additional strain on the fiber Bragg grating as the ambient temperature changes. Additionally, the fiber Bragg grating may be stripped of any external coating or buffer to eliminate the potential strain effects from the external coating and buffer. The anchoring may be achieved either by collapsing a glass tube over the entire length of the fiber Bragg grating, or by simply locally collapsing a glass tube at two ends of the fiber Bragg grating so that the glass tube surrounds and encases the fiber Bragg grating. The glass tube is then held in a fixture which does not permit strain into the tube and the fiber Bragg grating therein. The fixture is achieved by attaching one end of the tube or glass element containing the fiber Bragg grating to a reference housing, leaving the other end free to move. Additionally, the glass element does not contact the reference housing other than the contact at the attached end. This is necessary to assure that over a temperature change the differences of the coefficient of thermal expansion between the two materials do not cause friction or sticking which would cause the fiber Bragg grating to strain and produce an error term. Good heat conduction is also maintained between the reference housing and the fiber Bragg grating by minimizing all air gaps. Where an air gap is required, the depth and length should be minimized to reduce the insulating capacity of the cavity. In addition, because the attachment end of the fiber Bragg grating should be in as much contact with the reference housing as possible, only a thin layer of epoxy holds the fiber Bragg grating. This will facilitate the movement of heat from the reference housing into the fiber Bragg grating and ensure that the fiber Bragg grating and the reference housing maintain the same temperature. To accurately correlate the temperature of the fiber Bragg grating and the temperature measured externally, a temperature probe such as a thermistor should be located as closely as possible to the fiber Bragg grating and in contact with the reference housing.
In order to minimize the effects of a temperature differential to exist between the fiber Bragg grating and the thermistor, particularly when the environmental temperature is rapidly changing, good thermal conductive and insulating layers are used to surround the fiber Bragg grating and thermistor. For example, the fiber Bragg grating and thermistor are embedded in a good thermal conductive material, such as aluminum. This conductive material acts to rapidly distribute the heat present in the block equally throughout the block, and prevent thermal gradients from occurring. Besides an insulating layer is placed around the thermal conductive material (also known as a thermal mass block) to prevent rapid temperature changes from exceeding the heat distribution capabilities of the block and creating a thermal gradient across the entire block. By effectively increasing the time constant for the thermal block, a rapid temperature change in the environment will not cause a differential temperature between the fiber Bragg grating and the thermistor.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing, which are not drawn to scale.